Introduction

Heavy metal contamination is a major environmental concern affecting ecosystems (Gray et al., 2006). More than five million sites, including agricultural lands worldwide, have been contaminated with various heavy metals (Liu, et al., 2013; Palansooriya et al., 2020). The non-biodegradability and toxicity of heavy metals in soils have severely impacted ecological functions, food security, and human health (Rai et al., 2019; Fei et al., 2022). Heavy metals in soils can be categorized into water-soluble, exchangeable, reducible, oxidizable, and residual fractions (Lee et al., 2006; Ali et al., 2024; Kim, 2024; Wu et al., 2024). The available fraction is the most critical, with water-soluble and exchangeable metal concentrations being particularly bioavailable (Weerasundara et al., 2017).

Plants absorb available heavy metals from the soil through their roots, store them in stems and leaves, and transfer them to edible parts (Zhou et al., 2019; Kim et al., 2021). Metal concentrations in different parts of plants vary, as each plant species possesses specific mechanisms for the uptake, translocation, and accumulation of trace metals (Madejon et al., 2003; Kim et al., 2019; Sekara et al., 2005; Tangahu et al., 2011; Kim et al., 2021). Heavy metals such as cadmium, chromium, copper, lead, nickel, and thallium have been reported to exert significant effects on plant growth and seed germination, although their impacts may vary depending on metal type and plant species (Iyaka, 2011; Banjac et al., 2021; Espinosa et al., 2023; Chang et al., 2024). In particular, Tl is widely distributed in the environment, including agricultural soils near cement plants, abandoned mines and smelter areas in Korea with concentrations ranging from 0.18 to 12.91 mg kg^-1 (Lee et al., 2015; Kim et al., 2021; Kang et al., 2023).

Mallow is a widely cultivated vegetable used as a cooking ingredient in Korean cuisine, recognized for its high dietary fiber content, which supports bowel function and helps alleviate constipation. Additionally, it is classified as an antioxidant-rich medicinal plant and has shown potential therapeutic activity against respiratory disorders and inflammation-related diseases (Barros et al., 2010; Akbar et al., 2014; Rhimi et al., 2025). Mallow species also have the ability to accumulate various heavy metals, including lead, cadmium, copper, nickel, iron, manganese, zinc, and cobalt (Galal et al., 2019). The mallow plant has been cultivated alongside various crops across a wide range of upland fields, including areas contaminated with heavy metals.

Therefore, the aim of this study was to investigate the uptake and translocation of selected heavy metals, including cadmium (Cd), nickel (Ni), lead (Pb), and thallium (Tl), in mallow (Malva verticillata L.) cultivated in a hydroponic system contaminated with the target metals.

Materials and Methods

Plant and growing procedure

Mallow (Malva verticillata L.) seeds were obtained from Asia Seed Korea. They were first soaked in distilled water for three hours, then placed between moist filter papers (Advantec #6, 110 mm) inside Petri dishes and left overnight in darkness. After sprouting, the seeds were transferred to seedbed pots containing sterilized carouse sand and supplemented with a nutrient solution containing the following compounds per liter: 150 mg of KCl, 120 mg of MgSO₄, 946 mg of Ca(N0₃)₂·4H₂0, 68 mg of KH₂PO₄, 0.06 mg of ZnSO₄·7H₂0, 0.69 mg of H₃BO₃, 0.017 mg of CuCl₂·2H₂0, 0.024 mg of Na₂MoO₄·2H₂O, 0.022 mg of MnCl₂·4H₂O, and 0.6 mg of FeCl₃ (Garland et al., 1981; Lee et al., 2002; Kim et al., 2016) and they were grown in a greenhouse plant nursery until their shoots reached approximately 15 cm in height.

Well-grown mallow seedlings were collected from the seedbed pots. Their roots were thoroughly washed under running tap water to remove any attached seedbed materials, then rinsed with distilled water and soaked in a nutrient solution for 24 hours. Three replicates of the seedlings were then transferred to separate rectangular polypropylene growth containers, each treated with a nutrient solution containing one of the target metals including Cd, Ni, Pb, or Tl at concentrations of 0, 5, 15, 30, and 50 mg L^-1, using CdCl₂, NiCl₂, PbCl₂, and TlCl solutions, respectively.

The greenhouse plant nursery was maintained at a temperature of 20 - 25°C with a 14/10-hour light/dark cycle (80 - 90 Klux) and 60 - 80% humidity. A nutrient solution free of heavy metals was routinely added to replenish nutrients and compensate for water lost through evapotranspiration. The plant was allowed to grow in the labeled solution for seven days with a continuous air supply.

Determination of heavy metals

After harvesting, the plant roots were thoroughly washed under continuously flowing water, rinsed with deionized water, and blotted with laboratory paper towels to remove excess moisture. The plants were then separated into shoot and root parts, dried in a drying oven at 80°C for 48 hours, and weighed. Samples (0.5 g) of each dried shoot or root were transferred into a 100 mL beaker, to which 5 mL of concentrated HNO₃ was added. The beakers were covered with a watch glass and left to stand overnight. The samples were then digested by adding 3.0 mL of 30% H₂O₂ and heating on a hot plate at 125°C until the digest became clear. Once clear, the watch glass was removed, and the digest was allowed to evaporate to near dryness at 80°C (Jones, 1991). The analysis of heavy metals in the digested solutions was conducted using inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 7300DV, PerkinElmer, USA).

Bioconcentration factor and translocation index

The bioconcentration (or accumulation) factor (BCF) and translocation index (TLI, %) of the heavy metals were calculated using the following equations (Zayed et al., 1998; Lee et al., 2002; Ekvall and Greger, 2003; Marchiol et al., 2004; Hladun et al., 2015; Kim et al., 2016; Kim et al., 2017): BCF = (M-ST or M-RT) / M-NS, where M-ST and M-RT are the target metal concentrations in the shoot and root, respectively, and M-NS is the target metal concentration in the nutrient solution, and TLI (%) = [(M-ST)/ (M-ST+ M-RT)] × 100.

Statistical analysis

Statistical analyses were conducted using SPSS (Statistical Package for the Social Sciences, version 20, SPSS Inc., Chicago, IL, USA). The significance of the parameters was determined using one-way analysis of variance (ANOVA), followed by Duncan’s Multiple Range Test. Correlations among the target metal concentrations in the plant, bio-concentration factor (BCF), and translocation index (TLI) were assessed using correlation analysis. Data for heavy metal concentrations, BCF, and TLI were tested for normality using the Shapiro–Wilk test (p < 0.05). Since the data followed a normal distribution, Pearson’s correlation analysis was used to calculate correlation coefficients (r).

Results and Discussion

Uptake and translocation of cadmium (Cd), nickel (Ni), lead (Pb), and thallium (Tl) in mallow plants grown hydroponically under varying concentrations of these metals are presented in Table 1.

The concentrations of Cd, Ni, Pb, and Tl in both shoots and roots of the mallow plants increased with increasing levels of the respective metals in the hydroponic nutrient solution. However, the uptake and translocation patterns varied among the four heavy metals. Cd concentrations in shoots ranged from 55.3 to 371.4 mg kg^-1 and in roots from 185.2 to 1908.0 mg kg^-1. Ni concentrations ranged from 78.2 to 971.1 mg kg^-1 in shoots and from 155.1 to 1211.7 mg kg^-1 in roots. Pb concentrations were between 21.1 and 67.6 mg kg^-1 in shoots and between 225.5 to 2489.7 mg kg^-1 in roots. Tl concentrations ranged from 62.0 to 582.5 mg kg^-1 in shoots and from 289.1 to 2212.2 mg kg^-1 in roots. The order of metal uptake concentrations was Ni > Tl > Cd > Pb in the shoots, and Pb > Tl > Cd > Ni in the roots, except at 5 mg L^-1 of Pb and Tl application, where Tl > Pb. However, Sanjosé et al. (2022) reported different results for the glasswort plant. The order of metal concentrations in glasswort was Pb > Tl > Ni > Cd in the leaves, Pb > Ni > Tl > Cd in the stems, and Pb > Ni > Cd > Tl in the roots. These findings suggest that heavy metal uptake and translocation may depend on the physiological mechanisms of each plant species in response to specific metals. A wide range of plant species is capable of absorbing and accumulating heavy metals in various vegetative and generative organs, and metal translocation can vary depending on the development stage of the plants (Nescu et al., 2022). Nonetheless, the concentrations of heavy metals in the roots of various plants were consistently higher than those in the shoots, indicating that plant roots have a high capacity to adsorb metal ions from the soil solution surrounding the root zone (Dushenkov et al., 1995; Tangahu et al., 2011). Also, Rahman et al. (2024) reported that Pb generally accumulates in the root cells after entering through the roots, while a very small portion is translocated to the shoot due to natural endodermis barriers. A large portion of the metal fraction in plant roots appears to be localized in the apparent free space (AFS) in a nonmetabolic reaction (Verkleij and Schat, 1990). This is because cell walls are typically negatively charged due to the abundance of free carboxyl groups in the pectins of the middle lamella and primary wall. As a result, ion movement within the cell wall is governed by electrostatic interactions, leading to the accumulation of cations (Marschner, 1995).

On the other hand, the bioconcentration factor (BCF) ‒ including BCF-ST (for shoots) and BCF-RT (for roots) ‒ and the translocation index (TLI) of each heavy metal in mallow showed different trends. BCF-ST values for Cd and Pb decreased with increasing concentrations of the applied target metals, whereas BCF-ST values for Ni tended to increase. In contrast, BCF-ST values for Tl showed no significant change or exhibited a slight decreasing trend. However, only the BCF-RT values for Tl declined with increasing the metal concentrations, while the BCF-RT values for Cd, Ni, and Pb increased with rising concentrations of their respective metal treatments, except at the highest dose (50 mg L^-1), where they decreased. The orders of mean BCF-ST and BCF-RT values (n = 4, each with three subsamples) corresponded to the metal uptake concentrations in the shoots and roots, respectively. In addition, TLI values for Cd and Pb tended to decrease with increasing concentrations of the respective metals, whereas TLI values for Ni and Tl showed increasing trends with rising concentrations of the applied metals. The order of mean TLI values (%, n = 4) in mallow was Ni > Tl > Cd > Pb, reflecting a trend similar to that of metal concentrations in the shoots.

Davis et al. (2023) examined the uptake and translocation of Cd and Pb in bush morning glory, water-hyacinth, and six other plants, showing that BCF and BCF-ST/-RT values increased with rising concentrations of the applied metals, whereas TLI values for both metals decreased under the same conditions. Mohtadi and Schat (2024) investigated Ni translocations in three plants, including the Ni hyperaccumulator Odontarrhena corsica and the non-hyperaccumulator Aurinia saxatilis and Lobularia maritima. TLI values for Ni in Odontarrhena Corsica and Lobularia maritima increased with higher concentrations of applied Ni, while those in Aurinia saxatilis decreased with increasing metal concentrations. In addition, in a previous study on Tl, BCF-ST and TLI values in barley and cucumber decreased, while BCF-RT values increased with rising metal concentrations in the nutrient solution. In contrast, both BCF-ST and BCF-RT values in sunflower declined, but TLI values in sunflower increased with increasing Tl concentrations (Kim et al., 2016 and 2021). These results suggest that metal uptake and translocation in plants depend on the specific interaction between the metal and the plant species. In particular, considering its limited ability to take up and translocate Pb, mallow may be relatively safer as a food crop with respect to Pb contamination compared to Cd, Ni, and Tl.

The correlation coefficients (r) among shoot (ST) and root (RT) heavy metal concentrations, bio-concentration factors (BCF-ST and BCF-RT), and the translocation index (TLI) for each target metal in mallow are shown in Table 2.

The concentrations of Cd and Ni in the shoots of mallow were positively correlated with those in the roots, with significance levels of p < 0.05 and p < 0.1, respectively. BCF-ST values for Cd and Ni were also positively correlated with their respective TLI values (p < 0.1 for both metals). Pb concentrations in the shoots were positively correlated with those in the roots (p < 0.05), and BCF-ST values for Pb were positively correlated with its TLI values (p < 0.05). However, Pb concentrations in the roots were negatively correlated with BCF-ST values (p < 0.1). In the Tl-contaminated system, Tl concentrations in the shoots were significantly positively correlated with those in the roots (p < 0.01). Unlike the Cd-, Ni-, and Pb-contaminated systems, BCF-ST values for Tl were not correlated with its TLI values. Tl concentrations in both the shoots and roots were negatively correlated with BCF-RT values (p > 0.05), but were significantly positively correlated with TLI values (p > 0.01). Additionally, BCF-RT values for Tl were negatively correlated with TLI values (p > 0.05). Nonetheless, significant correlations were not observed among the other parameters in the Cd-, Ni-, Pb-, and Tl-contaminated plant–solution systems. These statistical results indicated that all metal concentrations in the shoots of mallow were significantly correlated with those in the roots. The total concentrations of Cd, Ni, and Pb in whole plant tissues were also significantly correlated with their respective concentrations in the nutrient solution, reflecting their accumulation in the shoots. However, in the case of Tl, the total Tl concentration in plant tissues was not significantly correlated with its concentration in the nutrient solution, indicating a different pattern of shoot accumulation.

The correlation coefficients (r) among Cd, Ni, Pb, and Tl in relation to BCF-ST, BCF-RT, and TLI values in mallow are presented in Table 3.

BCF-ST values for Pb were positively correlated with those for Tl (p < 0.1), and TLI values for Cd were also positively correlated with those for Pb (p < 0.1). In contrast, BCF-ST and TLI values between the other metals were not significantly correlated. Furthermore, no significant correlations were observed among BCF-RT values for any of the metals. These results indicate that the pattern of Pb accumulation in the shoots of mallow is statistically similar to that of Tl, and that the translocation trend of Cd may resemble that of Pb, even though TLI values for Pb were markedly lower than those for Cd.

Conclusions

This study investigated the uptake and distribution of cadmium (Cd), nickel (Ni), lead (Pb), and thallium (Tl) in Malva verticillata L. under hydroponic conditions with increasing metal concentrations. All four metals accumulated in both shoots and roots as exposure levels rose, but patterns differed by metal. Ni and Tl were more concentrated in shoots, whereas Pb mainly remained in roots. BCF and TLI values showed distinct trends across metals, reflecting their varying distribution within the plant. Cd and Pb showed decreasing TLI values with higher concentrations, while Ni and Tl showed increasing trends. Correlation analysis confirmed consistent relationships between shoot and root levels, though Tl showed less predictable behavior compared to the other metals. Overall, metal-specific differences were observed in uptake and distribution. Mallow showed relatively limited accumulation and movement of Pb, suggesting it may pose lower concern for Pb contamination when cultivated as a food crop.